In laboratories which are occupied with molecular- biological/biochemical research, the fields of “genomics” or “proteomics” are commonly used terms for processing and research into idioplasm, such as DNA (deoxyribonucleic acid), RNA (ribonucleic acid), or their parts in the form of oligonucleotides or of proteins (albumins, for example in the form of antigens or antibodies, or their parts in the form of polypeptides). Such and similar processes can comprise a multitude of work steps at various work stations. The field of proteomics in particular is gaining in importance, because not only the genome (idioplasm), but primarily the respectively provided protein setup (proteome), determine the appearance and the state of a biological organism. This knowledge led to the necessity that today a deeper understanding of proteins as the actual regulation networks taking the place of the dogma “one gene—one protein—one function”.
Proteomics—the quantitative analysis of the proteins present in an organism at a defined time and under defined conditions—will therefore distinguish itself as an important key for functional analysis in basic research (for example in connection with the elucidation of reaction and regulation networks), as well as in applied research (for example in connection with the search for and selection of targets for the development of medicaments).
Systems which are capable of performing automated separation, or separation and cleaning methods, typically employ so-called “SPE plates” (Solid Phase Extraction plates) for processing samples, in particular for solid phase extraction. The principle of solid phase extraction can be summarized as follows: A sample is applied to a solid sorbent, which adsorbs or binds defined components of the sample. These components are often called target molecules, however such components might be non-ionic as well as ionic species or particulate matter like cells, cellular substructures like mitochondria or nuclei, or virus particles. In the following disclosure the term “particles” is understood to cover all types of target molecules mentioned above. In performing solid phase extraction after this binding step the remainder of the sample is separated from the solid sorbent. The next step most commonly applied is washing of the solid phase loaded with the particles. Finally the particles are eluted from the solid sorbent. The resulting fluid contains a purified and/or concentrated fraction containing the particles, e.g. the target molecules. In the process—depending on the target of the application—a sorbent, e.g. a specifically activated filter or an appropriate screen is placed in, or at least close to, the bottom outlet opening of a small cup or “well” of a microplate (see FIG. 1). To perform a separating process, a sample is pipetted into a well and is forced to leave the well of the microplate through the sorbent via the bottom outlet opening. Typically suction forces (by the application of a vacuum) or positive pressure or gravity (e.g. by means of centrifuging) are used for this purpose.
In the course of this process, the target molecules therefore adsorb or bond to the activated material. After performing some washing steps, the target molecules, or the organic, or inorganic particles separated from the sample as described above, can be eluted with the aid of an eluent (a suitable solvent), i.e. separated from the filter, or the screen. The eluted particles are subsequently transferred onto a second microplate or to the surface of a support as described above.
Usually the sorbents used as packings in solid phase extraction comprise a base material, which should be inert to the sample components, and binding groups on the base material. These binding groups bind specifically the particles, e.g. target molecules. Sometimes the base material itself binds the target molecules, e.g. nucleic acids are reversably bound by silica (SiO2). Porous or non-porous particulate materials or other porous formed bodies made of organic or inorganic compounds usually form the base material. Examples of organic polymers are styrene-divinylbenzene copolymers or hydrophilic copolymers based on poly-(meth)acrylates, or polyamides. Porous membranes, fibrous materials, e.g. woven or non-woven fabrics or felts can also be used as organic base materials. Typical examples of inorganic base materials are metal oxides, especially SiO2 or Al2O3. The binding groups can be directly introduced to the base material, e.g. by sulfonating aromatic groups of styrene-divinylbenzene copolymers. Binding groups can also be introduced to the base material by polymerizing suitable monomers onto the base material. Inorganic base materials can be modified using organic substituted silanes containing e.g. ionic groups. Sorbents usable in solid phase extraction are basically available commercially or are described in the literature.
Often the filters, or screens, employed with known SPE plates have different flow resistances so that—in case a vacuum was used for emptying the SPE plates—some wells are emptied more rapidly than others. Then the emptying of all wells can only be achieved by the sudden application of a strong vacuum. But this often leads to a spraying, or even foaming, eluate, which can lead to undesired material transfer to neighboring wells, or to a loss of the sample.
WO 98/37 949 discloses a pipette tip which contains a sorbent. Said sorbent consists of a composite casted in place, whereby said composite consists of a plurality of sorptive particles entrapped in a porous polymer matrix.
Also known are so-called “ZipTips™”, which are sold by the Millipore firm (Millipore Corporation, 80 Ashby Road, Bedford, Mass. 01730-2271, USA)(see FIG. 2). Such “throwaway tips” are placed on a pipette and have a large interior of approximately 10 to 20 μl. If the tips are not completely filled with fluid, an air cushion remains between the fluid surface and the piston of the pipette. This air cushion acts like a damping element during aspirating (picking up) and dispensing (releasing) of the fluid. For this reason the determination of the end point of picking up and releasing fluids—in particular if only small volumes are intended to be pipetted—requires great skill and effort, if it can be reproduced at all.
Features necessary for improving sorbents in solid phase extraction are:                low unspecific binding of sample components to the base material;        high binding capacity including dynamic binding capacity at high linear flow rates (the latter describes the loss of binding capacity at elevated linear flow rates);        low hydrodynamic resistance to allow high linear flow rates at low pressure drop;        especially in microplate assemblies the hydrodynamic resistance of the sorbent in each well should vary little between different wells.        